1. Field of the Invention
The invention relates to a threading apparatus for threading the start of a printing-material web using a threading point in a web-fed rotary printing machine and a threading method for threading the start of a printing-material web.
2. Description of the Related Art
In web-fed rotary printing machines for printing printing-material webs such, for example, as paper webs it is occasionally necessary to laterally offset the printing-material web. The lateral offsetting is required, for example, when printing is being carried out with a printing-material web which covers only the left or the right half of the cylinders or which is only one quarter of the possible width of a four-page-width printing machine. If the half-width printing-material web is intended to be folded over a folding fonner which is arranged centrally in the middle of the full-width printing-material web, then it is necessary for the half-width printing-material web to be offset by one quarter of the width of the machine to the right or to the left, for which purpose a pair of turner bars arranged parallel to each other is used.
In another situation which requires lateral offsetting, printing-material webs which occupy the full width of the cylinders may be cut in the longitudinal direction by a longitudinal cutting device into two halves with the requirement that both halves of the full width printing-material web are then to be laid one above the other so that they are folded together in the longitudinal direction via a folding former. To place the halves of the full length printing-material web one above the other, each half must be offset toward the center, if the folding former is arranged centrally. Likewise, however, it is also conceivable that only one of the two part printing-material webs is offset until it runs above or below the other part printing-material web.
When a printing-material web is required to be laterally offset in relation to its original running direction by offset bars, difficulties arise with regard to the use of a threading apparatus. Normally, in these cases threading apparatuses thread the printing-material webs only as far as the region of the offset bars. The automatic threading is then stopped, the leading end of the printing-material web is removed from the threading point of the threading apparatus and is led further manually through the region of the turner bars.
DE 44 09 693 C1 discloses a prior art apparatus in a web-fed rotary printing machine for threading part printing-material webs which are produced from a single printing-material web by a longitudinal cutting device, in which the part printing-material web produced in the longitudinal cutting device runs further over offset bars, being threaded during the threading operation by a flexible threading point fastened to a lateral sheet chain. The lateral sheet chain has rollers and is also movable along the longitudinal axis direction of the rollers. The guide rail for the part printing-material web running over the offset bars runs outside the displacement or pivoting region of the offset bars, to the side of the running path of the part printing-material web. The construction disclosed by DE 44 09 693 C1, which enables the automatic threading of the part printing-material web through the offset bars, is a complicated construction and permits automatic threading only if one and the same part printing-material web is to be laid above the other part printing-material web and if the same lateral offset is always desired.